Electron shift, redox-induced

The reduction pattern of Ru6 warrants some additional comments it can be considered that the first electron probably enters the inner 2,3-dpp ligand, which should be the easiest site to be reduced however, upon second electron addition to one of the outer bridging ligands, the first electron should move to one of the other outer bridges, for coulombic reasons. This sort of redox-induced electron shift is not rare in this class of metal-polypyridine dendrimers.38... 

Another method used to vary the AG° of the recombination reaction without chemical modification of the centers, consists of placing the system in an electric field whose orientation and intensity are well defined [141]. However, the energy level shifts induced by the field also change the electronic factors, so that the interpretation of the experimental results is not straightforward. Bixon and Jortner have proposed using electric field effects to elucidate the nature of the primary electron step in bacterial photosystems [142], a problem that will be discussed in Sect. 3.5. One basic difficulty encountered in this method is the evaluation of the internal field effectively seen by the redox centers in the membrane. 

Studies on the electrochemical behavior of ferrocene encapsulated in the hemi-carcerands 61 and 62, indicated that encapsulation induces substantial changes in the oxidation behavior of the ferrocene subunit [98]. In particular, encapsulated ferrocene exhibits a positive shift of the oxidation potential of c. 120 mV, probably because of the poor solvation of ferrocenium inside the apolar guest cavity. Lower apparent standard rate constants were found for the heterogeneous electron transfer reactions, compared to those found in the uncomplexed ferrocene under identical experimental conditions. This effect may be due to two main contributions (i) the increased effective molecular mass of the electroactive species and (ii) the increased distance of maximum approach of the redox active center to the electrode surface. 

Zaban and co-workers reported the use of chemical redox titrations to measure the potential of sensitizers bound to Ti02 [136], An unexpected result from these studies is that redox couples that are not pH sensitive in fluid solution become pH dependent when bound to the semiconductor surface. The magnitude of the pH-induced shift varied from 21 to 53 mV per pH unit depending on the physical location of the sensitizer. Sensitizers inside the semiconductor double layer track the 59 mV pH shift of the semiconductor. When sensitzers were outside the double layer, their potential was almost independent of the semiconductor. This finding has important implications for the determination of interfacial energetics for dye sensitization and interfacial electron transfer studies [136]. 

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